A simple answer to a complicated question but the theory behind these diets is that slower-burning energy sources--protein and fat (nutrient dense foods)provide a steady supply of energy and thus are less likely to lead to weight gain. In addition, people tend to feel full(satisfied) longer after eating protein and/or fats than after eating carbohydrates, because carbohydrates empty from the stomach quickly and are digested quickly. Carbohydrates also strongly stimulate insulin( the fat hormone) production, which promotes fat deposition and increases appetite Plus high fat diets will incur ketosis much easier then high protien! Hope I made it a little clearer for you! Grog